rmcoen
Adventurer
I think "GM Agency" is the GM choosing to break rules / change results. And is 75% a Bad Thing (tm). [I know some people disagree, I've seen the "I decide when it is most dramatic for the monster to die" reddit threads...]
Example 1:
The party faces a Red Dragon. They prepare ahead of time, drinking potions of fire resistance. The red dragon breathes on them, and then all take full damage because the GM decided (on the spot) "it's infernal fire, and bypasses resistance." GM Agency. Probably a Bad Thing, unless he pulls a fantastic explanation out of his... um, out of thin air, and makes an amazing new storyline to follow because of it. (Still Bad, but mitigated.)
Example 2:
I built a custom monster for the party to face, based on the campaign Story, the Three-Horned Beast. It has various thematic resistances based on what it is, where it's been trapped, and the mutations/body-horror experiments performed on it. In play, I discovered that it was WAY TOO HARD -- I wanted it to be a legendary boss monster (and there was ample opportunities to run and come back later or never!), but damn, I overestimated. So I nerfed the Regeneration ability hard, and had the Beast's priorities change when it reached a certain already predefined stage (1/3 its health). GM Agency.
Example 3:
The Goblin Boss is supposed to escape the encounter while his minion goblins delay the party, so the BBEG in the next encounter has time to set up the set-piece event. The PCs pull some craziness, and the GM decides to let the craziness happen, knowing the Goblin Boss can handle anything, even a critical hit. And the critical hit happens, and then PCs' abilities allow them to finish off the Boss before it can run... The GM decides "the noise is enough to warn the BBEG", and still has the set-piece happen anyway, despite there being no scripted warning. GM Agency.
Example 1:
The party faces a Red Dragon. They prepare ahead of time, drinking potions of fire resistance. The red dragon breathes on them, and then all take full damage because the GM decided (on the spot) "it's infernal fire, and bypasses resistance." GM Agency. Probably a Bad Thing, unless he pulls a fantastic explanation out of his... um, out of thin air, and makes an amazing new storyline to follow because of it. (Still Bad, but mitigated.)
Example 2:
I built a custom monster for the party to face, based on the campaign Story, the Three-Horned Beast. It has various thematic resistances based on what it is, where it's been trapped, and the mutations/body-horror experiments performed on it. In play, I discovered that it was WAY TOO HARD -- I wanted it to be a legendary boss monster (and there was ample opportunities to run and come back later or never!), but damn, I overestimated. So I nerfed the Regeneration ability hard, and had the Beast's priorities change when it reached a certain already predefined stage (1/3 its health). GM Agency.
Example 3:
The Goblin Boss is supposed to escape the encounter while his minion goblins delay the party, so the BBEG in the next encounter has time to set up the set-piece event. The PCs pull some craziness, and the GM decides to let the craziness happen, knowing the Goblin Boss can handle anything, even a critical hit. And the critical hit happens, and then PCs' abilities allow them to finish off the Boss before it can run... The GM decides "the noise is enough to warn the BBEG", and still has the set-piece happen anyway, despite there being no scripted warning. GM Agency.